Method and system of depth triggers for marine geophysical survey cable retriever systems

ABSTRACT

Depth triggers for marine geophysical survey cable retriever systems. At least some of the illustrative embodiments include causing a submerged geophysical survey cable to surface. In some cases, the causing the cable to surface may include: fracturing a frangible link wherein the frangible link, before the fracturing, affixes position of a piston within a cylinder bore of a housing coupled to the geophysical survey cable, and the fracturing of the frangible link responsive to pressure exerted on a face of the piston as the geophysical survey cable reaches or exceeds a predetermined depth; moving the piston within the cylinder bore; and deploying a mechanism that makes the geophysical survey cable more positively buoyant.

BACKGROUND

Marine survey systems are used to acquire data (e.g., seismic,electromagnetic) regarding Earth formations below a body of water suchas a lake or ocean. The marine survey systems typically use a pluralityof sensor streamers which contain one or more sensors disposed within anouter jacket.

In some situations, one or more sensor streamers may be disconnectedfrom the survey system, the disconnection possibly caused by failure ofa coupling mechanism or in some situations the sensor streamer may besevered (e.g., by the propeller of a passing vessel). In some failurescenarios, particularly with sensor streamers filled with alcohol oroil, the sensor streamer becomes negatively buoyant, thus tending tosink. In order to avoid complete loss of the sensor streamer, aninflatable balloon system may trigger (i.e., a retriever system), whichcauses the sensor streamer to surface.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of exemplary embodiments, reference will nowbe made to the accompanying drawings in which:

FIG. 1 shows an overhead view of marine survey system in accordance withat least some embodiments;

FIG. 2 shows a perspective view of a retriever system in accordance withat least some embodiments;

FIG. 3 shows an exploded perspective view of a lifting bag system inaccordance with at least some embodiments;

FIG. 4 shows an exploded perspective view of ballast system inaccordance with at least some embodiments;

FIG. 5 shows a perspective cross-sectional view of a depth triggermechanism for a lifting bag system in accordance with at least someembodiments;

FIG. 6 shows a cross-sectional elevation view of a ballast system inaccordance with at least some embodiments;

FIG. 7 shows a method in accordance with at least some embodiments; and

FIG. 8 shows a cross-sectional elevation view of depth trigger mechanismin accordance with at least some embodiments.

NOTATION AND NOMENCLATURE

Certain terms are used throughout the following description and claimsto refer to particular system components. As one skilled in the art willappreciate, different companies may refer to a component by differentnames. This document does not intend to distinguish between componentsthat differ in name but not function. In the following discussion and inthe claims, the terms “including” and “comprising” are used in anopen-ended fashion, and thus should be interpreted to mean “including,but not limited to . . . .” Also, the term “couple” or “couples” isintended to mean either an indirect or direct connection. Thus, if afirst device couples to a second device, that connection may be througha direct connection or through an indirect connection via other devicesand connections.

“Cable” shall mean a flexible, axial load carrying member that alsocomprises electrical conductors and/or optical conductors for carryingelectrical power and/or signals between components.

“Rope” shall mean a flexible, axial load carrying member that does notinclude electrical and/or optical conductors. Such a rope may be madefrom fiber, steel, other high strength material, chain, or combinationsof such materials.

“Line” shall mean either a rope or a cable.

“About” shall mean plus or minus fifteen percent (15%) of the recitedvalue.

“Gas” in reference to a substance shall refer to the state of thesubstance at standard atmospheric pressure and temperature. The factthat a substance may be a liquid at certain pressures and/ortemperatures shall not obviate the substance's status as a gas.

“Non-triggered” with respect to a depth trigger mechanism or componentsthereof shall mean that the depth trigger mechanism is armed and has yetto change operational state from the armed condition.

“Triggered” with respect to a depth trigger mechanism or componentsthereof shall mean that the depth trigger mechanism has changedoperational state responsive to reaching or exceeding a predetermineddepth.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure or the claims. In addition, oneskilled in the art will understand that the following description hasbroad application, and the discussion of any embodiment is meant only tobe exemplary of that embodiment, and not intended to intimate that thescope of the disclosure or the claims is limited to that embodiment.

Various embodiments are directed to retriever systems for geophysicalsurvey cables used in marine surveys. More particularly, at least someembodiments are directed to depth trigger mechanisms for retrieversystems including lifting bag systems, ballast weight systems, or both.The ballast weight system enables the user to compensate the cable forbuoyancy changes caused by, for example, differences in salinity andtemperature of the water within which the marine survey will beperformed. However, the ballast weight system may also selectively shedor jettison the ballast weights when the survey cable reaches apredetermined depth indicative of potential loss. The ballast weightsystem may work in conjunction with a selectively deployable lifting bagsystem. The specification first turns to an illustrative marine surveysystem to orient the reader, and then to example embodiments.

FIG. 1 shows an overhead view of a marine survey system 100 inaccordance with at least some embodiments. In particular, FIG. 1 shows asurvey vessel 102 having onboard equipment 104, such as navigation,energy source control, and data recording equipment. Survey vessel 102is configured to tow one or more streamers 106A-F through the water.While FIG. 1 illustratively shows six streamers 106, any number ofstreamers 106 may be used. The discussion continues with respect tostreamers 106 being sensor streamers, but streamers 106 are illustrativeof any towed geophysical survey cable, such as transmitter cables andsource cables.

The sensor streamers 106 are coupled to towing equipment that maintainsthe streamers 106 at selected depth and lateral positions with respectto each other and with respect to the survey vessel 102. The towingequipment may comprise two paravane tow lines 108A and 1088 each coupledto the vessel 102 by way of winches 110A and 1108, respectively. Thewinches enable changing the deployed length of each paravane tow line108. The second end of paravane tow line 108A is coupled to a paravane112, and the second end of paravane tow line 1088 is coupled to paravane114. In each case, the tow lines 108A and 1088 couple to theirrespective paravanes through respective sets of lines called a “bridle”.The paravanes 112 and 114 are each configured to provide a lateral forcecomponent to the various elements of the survey system when theparavanes are towed in the water. The combined lateral forces of theparavanes 112 and 114 separate the paravanes from each other until theparavanes put one or more spreader lines 120, coupled between theparavanes 112 and 114, into tension. The paravanes 112 and 114 eithercouple directly to the spreader line 120, or as illustrated couple tothe spreader line by way of spur lines 122A and 122B.

The sensor streamers 106 are each coupled, at the ends nearest thevessel 102 (i.e., the proximal ends) to a respective lead-in cabletermination 124A-F. The lead-in cable terminations 124 are coupled to orare associated with the spreader lines 120 so as to control the lateralpositions of the streamers 106 with respect to each other and withrespect to the vessel 102. Electrical and/or optical connections betweenthe appropriate components in the recording system 104 and the sensors(e.g., 116A, 1168) in the streamers 106 may be made using inner lead-incables 126A-F. Much like the tow lines 108 associated with respectivewinches 110, each of the lead-in cables 126 may be deployed by arespective winch or similar spooling device such that the deployedlength of each lead-in cable 126 can be changed.

FIG. 2 shows a perspective view of a retriever system in accordance withat least some embodiments. In particular, FIG. 2 shows a portion of asensor streamer 106. The sensor streamer 106 has an elongated outerjacket 200 that defines an interior volume 202. The elongated outerjacket defines a central axis 204. Though not specifically shown in FIG.2, various sensors (e.g., hydrophones, geophones, electromagneticsensors) associated with the sensor streamer 106 reside within interiorvolume 202 and are spaced longitudinally along the sensor streamer 106.

FIG. 2 further shows a retriever system 206 in accordance with at leastsome embodiments. In particular, retriever system 206 comprises alifting bag system 208, and in some embodiments a ballast weight system210. While FIG. 3 only shows one retriever system 206, it will beunderstood that a sensor streamer may have a length on the order of lessthan 200 meters to in excess of 15000 meters, and a plurality of suchretriever systems 206 may be spaced along and thus associated with eachsensor streamer 106. As illustrated, a portion 212 of the elongatedouter jacket 200 may reside between the lifting bag system 208 andballast weight system 210, and the portion 212 may comprise one or moresensors. In some cases, the retriever systems associated with a sensorstreamer may be evenly spaced along the elongated outer jacket, andfurther the individual lifting bag systems and ballast weight systemsevenly spaced, but such even spacing is not strictly required. Thespecification first turns to the lifting bag system 208 in accordancewith various embodiments, and then turns to the ballast weight system210.

FIG. 3 shows an exploded perspective view of a lifting bag system 208(in a non-deployed condition) in accordance with at least someembodiments. In particular, the illustrative lifting bag system 208comprises a bag attachment block 300 that defines a first end 302 andopposite second end 304, both of circular cross-sections. The bagattachment block defines a plurality of passages 306 that extend betweenthe first end 302 and the second end 304 of the bag attachment block. Itis through the passages 306 that various electrical, load carryingmembers, and/or communicative conductors of the sensor streamer 106pass, such that power may be provided to the sensors and/or readingstaken from the sensors. The first end 302 and second end 304 define anoutside diameter (OD) sized to couple to an inside diameter of theelongated outer jacket 200 of the sensor streamer 106. In some cases,the first end 302 and second end 304 may comprise a plurality of grooves308 and 310, respectively, to assist in the coupling of the ends 302 and304 to the elongated outer jacket 200. The grooves may take any suitableform, such as rectangular grooves, triangular grooves, or groovessimilar to threads, just to name a few. The bag attachment block 300(including the ends 302 and 304) may be made from any suitable materialkeeping in mind that the buoyancy of the sensor streamer (with thelifting bag system 208 in a non-deployed state) is designed to beapproximately neutrally buoyant. Thus, the bag attachment block 300 maybe made from materials such as high density plastic, or light metalssuch as titanium or aluminum. Other materials, and combinations ofmaterials, may be also be used.

The lifting bag system 208 further comprises a bag 312. FIG. 3 shows thelifting bag system 208 with the bag in a deflated and stowed state. Whendeflated and stowed the bag 312 is folded such the amount of space usedto store the bag within the lifting bag system 308 is reduced. The bag312 in its inflated state may take any suitable shape, such as round orrectangular. When deployed, the bag itself may mechanically couple tothe bag attachment block 300 and support the weight of the sensorstreamer. In other cases, the bag may be held within a net or lattice ofropes mechanically coupled to the bag attachment block 300. The materialfrom which the bag 312 is constructed may take any suitable form. Insome cases, the bag 312 material may be a plastic material, plasticcoated fabric, or water tight or water resistant material.

In order to inflate the bag 312 when needed, the lifting bag system 208further comprises gas cylinder 314 coupled to the bag attachment block300. The gas cylinder 314 comprises a compressed gas that, whenselectively released by depth trigger mechanism 316, inflates the bag312. The compressed gas within the cylinder 314 may take any suitableform, such as compressed air, compressed nitrogen, compressed carbondioxide, or other gas. In at least some embodiments, the compressed gasis held at a pressure and temperature where the gas becomes a liquid.More particularly, in some embodiments the compressed gas in thecylinder 314 is liquid carbon dioxide.

The lifting bag system 208 further comprises a depth trigger mechanism316. When the depth of the lifting bag system 208 meets or exceeds apredetermined depth, the depth trigger mechanism 316 fluidly couples thecompressed gas from the gas cylinder 314 to the internal volume of thebag 312 such that the bag 312 inflates. Illustrative depth triggermechanism 316 defines an outer housing 318 into which a cylinder bore320 is created. Within the cylinder bore 320 resides a piston 322 whichis exposed to the ambient pressure of the water. The piston 322 is onlypartially visible in FIG. 3, the partial visibility caused by the coverplate 323, which cover plate is discussed more below. It is noted thatbeing exposed to the ambient pressure does not necessarily mean thepiston 322 is itself exposed to the water. Mechanisms for exposing thepiston 322 to the ambient pressure without directly exposing the pistonto the sea water are discussed more below. Generically stated, thetrigger mechanism 316 is a mechanical system where increasing depth(i.e., increasing ambient pressure) moves the piston 322, which movementpunctures a seal of the gas cylinder 314, which couples the compressedgas to the bag 312.

Still referring to FIG. 3, the lifting bag system 208 further comprisesouter cover 324. In some embodiments, the outer cover 324 is a singlefrangible unit designed and constructed to break away as the bag 312begins to inflate. Illustrative outer cover 324 is shown as comprisingtwo halves 326 and 328. The covers 326 and 328 may couple to each otherand/or a portion of the bag attachment block 300 as appropriate. In aparticular embodiment, the outer covers 326 and 328 are designed andconstructed to separate from each other as the bag 312 begins toinflate. In another embodiment, the outer covers 326 and 328 areassembled to form the overall outer cover 324 but may be frangible,breaking into smaller pieces as the bag 312 begins to inflate. The outercover may be made of any suitable material, such as a plastic material.

The retriever system 206 in accordance with at least some embodimentsfurther comprises a ballast system 210. FIG. 4 shows an exploded,perspective view of a ballast system 210 in accordance with at leastsome embodiments. In particular, the illustrative ballast system 210comprises a ballast attachment block 400 that defines a first end 402and opposite second end 404, both of circular cross-section. The ballastattachment block defines a plurality of passages 406 that extend betweenthe first end 402 and the second end 404. It is through the passages 406that various electrical and/or communicative conductors of the sensorstreamer 106 pass. The first end 402 and second end 404 define anoutside diameter (OD) sized to couple to an inside diameter of theelongated outer jacket 200 of the sensor streamer 106. In some cases,the first end 402 and second end 404 may comprise a plurality of grooves408 and 410, respectively, to assist in the coupling of the ends 402 and404 to the elongated outer jacket 200. The grooves may take any suitableform, such as rectangular grooves, triangular grooves, or groovessimilar to threads, just to name a few. The ballast attachment block 400(including the ends 402 and 404) may be made from any suitable materialkeeping in mind that the buoyancy of the sensor streamer is designed tobe approximately neutrally buoyant. Thus, the ballast attachment block400 may be made from materials such as high density plastic, or lightmetals such as titanium or aluminum. Other materials, and combinationsof materials, may be also be used.

Illustrative ballast attachment block 400 defines a first attachmentlocation 412 and a second attachment location 414. In the illustrativeembodiments of FIG. 4, the attachment locations are not necessarilystructurally defined, except in relation to the depth trigger mechanisms416 and 418, respectively (only the piston portion of depth triggermechanisms 416 and 418 visible in FIG. 4). In other cases, theattachment locations 412 and 414 may be structurally delineated, such asby grooves, indentions, and/or areas of reduced diameter of the ballastattachment block 400. Also visible in FIG. 4 are the link members 417and 419 associated with the depth trigger mechanism 416 and 418,respectively. The link members will be discussed more below.

The ballast system 210 further comprises a first ballast weight 420 anda second ballast weight 422. It is noted that while FIG. 4 shows thefirst ballast weight 420 in the upper orientation, and second ballastweight 422 in the lower configuration, though any rotational orientationof the weights is possible. Although FIG. 4 is an exploded view, in anoperational configuration the first ballast weight 420 abuts the ballastattachment block 400 at the first attachment location 412, and if usedthe second ballast weight 422 abuts the ballast attachment block 400 atthe second attachment location 414. In the illustrative embodiments ofFIG. 4, the ballast weights 420 and 422 mechanically couple to theballast attachment block 400 by way of their depth trigger mechanisms416 and 418, respectively. Example depth trigger mechanisms arediscussed move below.

The ballast weights may be constructed of any suitable substance. Forexample, in some cases the ballast weights are lead or bronze. In atleast some embodiments, each ballast weight 420, 422 weighsapproximately 1 kilogram. Thus, if both ballast weights are attached tothe ballast attachment block 400, the ballast system 210 may addapproximately 2 kilograms to the overall weight of the attached sensorstreamer. The ballast weights are added to ballast attachment blocksalong the length of a sensor streamer to adjust the buoyancy of thesensor streamer. That is, the sensor streamer may be designed andconstructed to be substantially neutrally buoyant in water of aparticular salinity and temperature. However, marine surveys may betaken in a variety of locations and a variety of local conditions, andthus the ballast weights may be added and/or removed at the surface tocompensate for the specific salinity and temperature of water expected.In cases where only one ballast weight is used at a particular ballastattachment block, a dummy cover, weighing substantially less than aballast weight, may be placed at the unused attachment location.

The specification now turns to various embodiments of the depth triggermechanisms, starting with the depth trigger mechanism for the liftingbag system 208. FIG. 5 shows a cross-sectional perspective view of adepth trigger mechanism 316 coupled to a gas cylinder 314 in accordancewith at least some embodiments. In particular, the depth triggermechanism 316 comprises outer housing 318. The outer housing may be madeof any suitable material, but copper-based alloys (e.g., brass,beryllium copper) offer better resistance to fouling by sea creatures,such as barnacles. A counter-bore within the outer housing 318 defines acylinder bore 500 within which the piston 322 is located. For a depthtrigger mechanism 316 designed to trigger at about 55 meters of depthand below, the cylinder bore 500 has an inside diameter of about 2.5centimeters (cm), but larger or smaller inside diameters may be used.

The outer housing 318 further defines another counter bore 502 withinwhich the neck 504 of the gas cylinder 314 may be inserted and coupled.In some cases, the inside diameter of the counter bore 502 may bethreaded, and likewise the outside diameter of the neck 504 of the gascylinder may be threaded, and thus the gas cylinder 314 couples to theouter housing 318 by way of a threaded connection 506. Other mechanismsto couple the gas cylinder 314 to the outer housing 318 may be used.

Still referring to FIG. 5, the piston 322 in accordance with theillustrated embodiments comprises a counter bore 508 within which afrangible link or link member 510 is coupled. As illustrated, thecounter bore 508 is internally threaded, and a portion of the linkmember 510 is externally threaded, such that the link member threadinglycouples to the piston 322. Other connection systems may be used. Asillustrated, the counter bore 508 is centered in the piston 322. Thatis, the counter bore 508 defines a central axis that is coaxial with thepiston central axis 512. Likewise, the link member 510 defines a centralaxis which is coaxial with the piston central axis 512. Other alignmentsare possible. As will be discussed more below, the link member 510 holdsthe piston 322 in a non-triggered state until depth of the depth triggermechanism reaches or exceeds a predetermined depth.

An outside diameter of the piston 322 seals against the inside diameterof the cylinder bore 500 by way of o-rings 514 within respective annulargrooves 516. While FIG. 5 shows two annular grooves 516 and two o-rings514, one or more o-ring and annular groove systems may be used. Whilethe o-rings 514 seal against the inside diameter of the cylinder bore500, the o-rings nevertheless enable movement of the piston 322 withinthe cylinder bore 500. That is, after the link member 510 has fractured,the ambient pressure against the outer face 520 of the piston 322 pushesthe piston 322 inwardly into the cylinder bore 500. Likewise, when thedepth trigger mechanism is triggered at depth, the pressure releasedwithin the cylinder bore 500 may tend to push the piston 322 outwardly.

The depth trigger mechanism 316 illustrated in FIG. 5 further compriseslance member 528 coupled to the piston 322 and disposed within the outerhousing 318. As the name implies, the lance member 528 is used to lanceor puncture the seal 530 of the gas cylinder 314 when the depth triggermechanism 316 reaches or exceeds the predetermined depth. While in somecases at least the outer face 520 and outside diameter of the piston 322is made of copper-based alloys to reduce fouling by sea creatures, thelance member 528 is made of hardened steel. As illustrated, the lancemember 528 couples to the piston 322 by way of a counter bore 533 in arod member 534. In some cases the counter bore 533 is internallythreaded, and the lance member 528 is externally threaded, and thus thelance member 528 couples by way of a threaded connection. Otherconnection mechanisms are possible. The lance member 528 defines asharpened point 536, which in the illustrative case of FIG. 5 is in theform of a spear. The lance member 528 may also be shaped in other formsto shear, puncture, and/or pierce open the seal of the cylindercontaining compressed gas.

The depth trigger mechanism 316 of FIG. 5 is shown in the non-triggeredstate (i.e., the lance member 528 has not punctured the seal 530). Thepiston 322 and lance member 528 are held in the non-triggered state bylink member 510. In particular, link member is coupled on one end to thepiston 322 (illustratively by way of the threads in counter bore 508).On the second end, link member 510 couples to the cover plate 323. Asillustrated, the cover plate 323 comprises an aperture 540 through whichthe link member 510 is telescoped. The illustrated aperture 540 definesa shoulder region 542 against which a head portion 544 of the linkmember 510 abuts after insertion. In other cases, the head portion 544may abut the outer face 546 of the cover plate 323. At depths above(i.e., more shallow) than the predetermined depth, the link member 510holds the piston 322 in the non-triggered state.

As the illustrative depth trigger mechanism 316 gets progressivelydeeper in water, the water pressure on the outer face 520 of the piston322 increases, while the pressure within the cylinder bore 500 behindthe piston stays relatively constant. The differential pressureexperienced by the piston 322 creates a tension force on the link member510. However, at depths above the predetermined depth the link memberhas sufficient mechanical strength to hold the piston 322 in thenon-triggered state. It is to be understood that the non-triggered stateis not characterized by a complete lack of movement of the piston 322into the cylinder bore 500. Though link member 510 in some embodimentsis metallic and rigid, some plastic deformation is possible. Thus, thenon-triggered state is characterized by the lance member 528 having yetto puncture the seal 530.

At the predetermined depth or below, the pressure exerted on the outerface 520 of the piston 322 creates a tension force that overcomes themechanical strength of the link member 510. In the embodimentsillustrated by FIG. 5, link member 510 is circular and has a reduceddiameter portion 548. At the predetermined depth and below, the tensionforce created by the ambient pressure acting on the piston face 520 andapplied to the link member 510 fractures link member 510. In someembodiments, the link member 510 may be made of brass, and the reduceddiameter portion 548 has an outside diameter of about 0.2 cm. With apiston 322 outside diameter of about 2.5 cm, the result is a fracturedepth for the link member of about 60 meters.

Other types of materials may be used for the link member 510.Copper-based alloys provide resistance to fouling from sea creatures,but in other embodiments any suitable material for the link member 510may be used. For example, in situations where the link member is notexposed to the sea water (discussed below) non-copper-based alloys maybe used, such as stainless steel. In yet still other cases, non-metallicmaterials may be used, such as plastics. The outside diameter will beadjusted according to the strength of the material from which the linkmember 510 is constructed and the trigger depth. The fracture mode,likewise, may be of any suitable type for the material used. Forexample, the fracture mode may be a ductile fracture involving someplastic deformation prior to fracture, or a ductile fracture with littleor no plastic deformation prior to fracture. It is further noted thatthe link member 510 need not define the reduced diameter portion 548, insome cases the entire link member 510 may have the outside diametercalibrated to fracture at the predetermined depth. Moreover, while linkmember 510 is described in terms of a member of circular cross-section(at least at portion 548), other cross-sectional shapes may be used.

When the fracture of the link member occurs, the depth trigger mechanism316 is placed in a triggered state where the piston 322 extends into thecylinder bore 500 a sufficient distance for the lance member 528 topuncture the seal 530. Once the seal is punctured, the gas in the gascylinder 314 is released and flows into the cylinder bore 500 and intothe air bag 312 (FIG. 3) by way of aperture 550.

While FIG. 5 shows only a single link member 510, multiple such membersmay be used. For example, in situations wherein triggering occurs atdeeper depths or in situations where the piston 322 has a larger outsidediameter, multiple link members may be used. In the illustrativeembodiments of FIG. 5, the tension force created by piston 322; however,the force applied to the link member 510 may cause a fracture by anysuitable physical process. For example, the link member may be arrangedsuch that the tension force applied to the link member may apply ashearing force to a portion of the link member 510, or the tension forceapplied to the link member may apply a compression force to a portion ofthe link member 510.

Cover plate 323 couples to the outer housing 318 and partially occludesthe aperture for the cylinder bore 500. In order to communicate theambient pressure to the outer face 520 of the piston 322, theillustrative cover plate 323 comprises at least one aperture 552. Thecover plate 323 may be made of any suitable material with sufficientstrength to hold the tension force placed on the link member 510. Insome cases, the cover plate 323 is constructed of a copper-based alloy(e.g., brass, beryllium copper) to reduce fouling by sea creatures, suchas barnacles.

The various embodiments discussed to this point have been with respectto the depth trigger mechanism used in connection with the lifting bagsystem 208, puncturing a seal of the gas cylinder when the systemreaches or exceeds the predetermined depth. However, the same principlesof operation may be used with respect to the ballast weight system 210,though no lancing operation is needed with respect to the ballastweights.

FIG. 6 shows a cross-sectional elevation view of a ballast system 210 inaccordance with at least some embodiments. In particular, FIG. 6 showsthe ballast attachment block 400 having the first ballast weight 420 andsecond ballast weight 422 in an abutting relationship with the ballastattachment block 400. The illustrated ballast attachment block 400differs from that illustrated in FIG. 4 in that the attachment locationsare more delineated, the delineation in the form of reduced outsidediameter portions 600 and 602. Moreover, FIG. 6 illustrates that thegrooves 408 and 410 may likewise define a smaller outside diameter thanmore distal portions of the first end 402 and second end 404,respectively. For example, the elongated outer jacket may telescope overand end 402 or 404, and then be held in place by a mechanical member,such as a band that circumscribes the elongated outer jacket at thelocation of the grooves.

Referring to ballast weight 420 as representative, ballast weight 420comprises a through bore 604, as well as an outside counter bore 606 oflarger diameter such that a shoulder region 608 is defined. The ballastweight 420 further comprises another counter bore 610 on an oppositeside of the through bore 604 from the counter bore 606. Illustrativedepth release mechanism 416 of FIG. 6 comprises retention plate 612,which may be attached to the ballast attachment block by way offasteners within threaded apertures 614 and 616. While FIG. 6illustrates the use of threaded fasteners, any suitable attachmentsystem may be used. The retention plate retains the piston member 618 atleast partially within a chamber 620. As illustrated, the chamber 620 isdefined within a chamber block member 623, but in other cases thechamber 620 may be defined directly by a counter bore in the ballastattachment block 400. The piston member 618 defines a retention portion622 that, in the illustrated non-triggered state, protrudes into thethrough bore 604.

Still referring to FIG. 6, the piston member further comprises chamberportion 626 that resides within the chamber 620. As illustrated, thechamber portion 626 seals against the inside diameter of the chamber620, for example, by way of o-rings 628 in respective grooves 630. Othersealing mechanisms may be used. Also within the chamber 620 is springmember in the form of a coil spring 632. The pressure of the gas (e.g.,air) within the chamber, as well as the force created by compression ofthe illustrative coil spring 632, tend to bias the piston member into anextended orientation, as shown by depth release mechanism 416.

The ballast weight 420 couples to the ballast attachment block, at leastin part, by way of frangible link or link member 634 and outer plate636. In particular, the outer plate 636 couples within the counter bore606 and abuts the shoulder region 608. The outer plate 636 definesapertures 638 such that the ambient pressure of the water can beconveyed to the piston 618. The outer plate 636 further comprises athrough bore 640 within which a counter bore 642 is created. The counterbore 642 creates a shoulder region against which a head of the linkmember 634 abuts. The opposite end of the link member couples to theretention portion 622 of the piston 618 in a similar manner as discussedabove with the respect to the depth trigger mechanism 316, and will notbe numbered in FIG. 6 so as not to unduly complicate the figure. Thelink member 634 in combination with the outer plate 636 hold the ballastweight 420 when the in the abutting relationship in operation (e.g., useduring a marine survey). Moreover, in embodiments where the retentionportion 622 extends into the through bore 604, interaction between theinside diameter of the through bore 604 and the retention portion mayprovide stability against movement of the ballast weight 420 along thelong axis of the ballast attachment block 400.

In the non-triggered state, the piston member 618 will tend be in itsmost extended orientation. As the ballast system 210 increases in depth,increased pressure (communicated at least in part through the apertures638 in the outer plate 636) creates a force tending to retract thepiston member 618 into the chamber 620. Both the gas within the chamber620 and the spring 618 tend to resist movement. Moreover, the linkmember 634 holds the piston 618 in the non-triggered state. It is notedthat some plastic deformation of the link member 634 is possible, andthus the non-triggered state is not characterized as a complete lack ofmovement of the piston.

However, when the ballast system 210 reaches or exceeds thepredetermined depth, the force applied to the piston member 618overcomes the various forces resisting movement of the piston member618. In a particular embodiment, the tension force applied to the linkmember 634 fractures the link member 634. Once the link member isfractured, the ambient pressure forces the piston 618 into a retractedorientation. In some cases, the retracted orientation withdraws theretention portion 622 out of the through bore 604. Once the link member634 is fractured, the ballast weight 420 is effectively released, andcan fall away.

While the depth release mechanism 416 is shown in the non-triggeredstate, depth release mechanism 418 is shown in the triggered state. Thatis, the link member 650 has been fractured, and the piston member 652 ofdepth release mechanism 418 is shown in a fully retracted orientation.In some embodiments, the depth trigger mechanisms of the ballast weightsystem 210 trigger at the predetermined depth being about 55 meters, butthe depth trigger mechanisms of the ballast weight system 210 maytrigger at depths above or below the lifting bag systems 208. In aparticular embodiment, each piston of the depth release mechanisms hasan outside diameter of about 2.5 cm, and the each link member has atleast a portion with an outside diameter of about 0.2 cm, but othersizes may be used. Moreover, in some cases the springs that biases thepistons outward may be omitted.

FIG. 7 shows a method in accordance with at least some embodiments. Inparticular, the method starts (block 700) and comprises causing asubmerged geophysical survey cable to surface (block 702). Causing thesurvey cable to surface may comprise: fracturing a frangible linkwherein the frangible link, before the fracturing, affixes position of apiston within a cylinder bore of a housing coupled to the geophysicalsurvey cable, and the fracturing of the frangible link responsive topressure exerted on a face of the piston as the geophysical survey cablereaches or exceeds a predetermined depth (block 704); moving the pistonwithin the cylinder bore (block 706); and deploying a mechanism thatmakes the geophysical survey cable more positively buoyant (block 708).Thereafter, the method ends (block 710).

In the embodiments of the depth trigger mechanism for the lifting bagsystem, the discussion has indicated that the pistons are exposed to thepressure of the water such that increasing pressure acts on the piston,eventually fracturing the link and moving from a non-triggered state toa triggered state where the seal on the gas cylinder is punctured. Insome cases, the piston is directly exposed to the sea water andpressure, but in other cases the piston may be shielded from the seawater, but nevertheless exposed to the increasing pressure. FIG. 8 showsa cross-sectional view of a portion of a depth trigger mechanism 800 todiscuss various embodiments of shielding the piston from the sea water.In particular, the depth trigger mechanism 800 comprises an outerhousing 802 that has counter bore 804 within which piston 806 islocated. The piston 806 defines a central axis 808 along which thepiston moves when the link member 810 is fractured. The depth triggermechanism 800 further comprises a resilient member 812 coupled over thecover plate 814. More particularly, the resilient member 812 is sealedagainst the outer housing 802 and/or the cover plate 814 such that theresilient member 812 fluidly isolates the volume 816 from the ambientsea water. The volume 816 defined by the inside surface 818 of theresilient member 812 and outer face 820 of the piston is filled with anon-compressible fluid, such as oil, filtered water, or alcohol. Theresilient material may be any suitable material, such as rubber orresilient plastic.

In operation, ambient pressure of the sea water presses against theouter surface 822 of the resilient member 812. Responsive to thepressure, the resilient material presses on the non-compressible fluidwithin the volume 816, which in turn presses against the outer face 820of the piston 806. In particular, the resilient member 812 presses onthe non-compressible fluid in the volume 816 by way of the apertures 824in the cover plate 814. The tension force created in the link member 810by the pressure eventually fractures the link member and causes a changein the operational state from non-triggered to triggered as discussedfor the various embodiments above. However, because the piston and linkmember are not exposed to the sea water, the piston and link member areless likely to experience fouling, such as by barnacles. Moreover, thechoice of materials for the piston and link member increases fromnot-only the copper-based alloys, but also stainless steels and evenhigh density plastics.

References to “one embodiment”, “an embodiment”, “a particularembodiment”, and “some embodiments” indicate that a particular elementor characteristic is included in at least one embodiment of theinvention. Although the phrases “in one embodiment”, “an embodiment”, “aparticular embodiment”, and “some embodiments” may appear in variousplaces, these do not necessarily refer to the same embodiment.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. For example, a retriever systemmay comprise just a lifting bag system, or just a ballast weight system.It is intended that the following claims be interpreted to embrace allsuch variations and modifications.

1. A method comprising: causing a submerged geophysical survey cable tosurface by fracturing a frangible link wherein the frangible link,before the fracturing, affixes position of a piston within a cylinderbore of a housing coupled to the geophysical survey cable, and thefracturing of the frangible link responsive to pressure exerted on aface of the piston as the geophysical survey cable reaches or exceeds apredetermined depth; and then moving the piston within the cylinderbore; and deploying a mechanism that makes the geophysical survey cablemore positively buoyant.
 2. The method of claim 1 wherein deploying themechanism further comprises puncturing a seal of a compressed gascylinder by movement of a lance, wherein movement of the lance isresponsive to movement of the piston.
 3. The method of claim 2 whereinlance is coupled to the piston.
 4. The method of claim 1 whereindeploying the mechanism further comprises releasing a ballast weight,wherein prior to the releasing the ballast weight is at least partiallyheld in place by the frangible link.
 5. The method of claim 1 whereinfracturing the frangible link further comprises applying tension to abrass rod coupled to the piston, the piston having a diameter of about2.5 centimeters (cm), and the brass rod having at least a portion thathas a diameter of about 0.2 cm.
 6. The method of claim 1 whereinfracturing the frangible link further comprises fracturing the frangiblelink by application of tension to the frangible link as the piston meetsor exceed the predetermined depth.
 7. The method of claim 1 wherein thepredetermined depth is about 55 meters.
 8. A system comprising: a bagattachment block defining two ends, and at least one end configured tocouple a geophysical survey cable; a lifting bag coupled to the bagattachment block, the lifting bag deflated; a gas cylinder coupled tothe bag attachment block, the gas cylinder storing a compressed gas andhaving a seal; and a depth trigger mechanism coupled to the bagattachment block and the gas cylinder, the depth trigger mechanismcomprising a housing defining a cylinder bore; a piston disposed withinthe cylinder bore, the piston defining an outer face exposed to ambientpressure; a lance member disposed within the housing; a link membercoupled to the piston, wherein at depths in water above a predetermineddepth the link member holds the piston in a non-triggered state; andwherein at the predetermined depth and below pressure acting on the faceof the piston creates a tension force that breaks the link member, movesthe piston to a triggered state, and causes the lance member to puncturethe seal of the gas cylinder.
 9. The system of claim 8 wherein the linkmember defines a central axis, the piston defines a central axis, andthe where the central axis of the link member is coaxial with thecentral axis of the piston.
 10. The system of claim 8 wherein the linkmember is a copper-based alloy.
 11. The system of claim 8: wherein thelink member is brass and has at least one portion defining an outsidediameter of about 0.2 centimeter (cm); and wherein the piston has anoutside diameter of about 2.5 cm.
 12. The system of claim 8 wherein thepredetermined depth is about 55 meters.
 13. The system of claim 8further comprising: a cover plate coupled to the housing; and anaperture in the cover plate; the link member defining a first endcoupled to the cover plate and a second end coupled to the piston. 14.The system of claim 13 further comprising: a resilient member abuttingthe cover plate; and a non-compressible fluid with a volume definedbetween the face of the piston and the resilient member.
 15. A ballastsystem comprising: a ballast attachment block comprising: a first endand a second end, each end defining an outside diameter configured tocouple to an inside diameter of a geophysical survey cable; a firstattachment location medially disposed on the ballast attachment block; apassage extending between the first end and the second end, the passagedisposed within the ballast attachment block; a first ballast weightabutting the first attachment point, the first ballast weight defining afirst aperture; a first depth release mechanism coupling the firstballast weight to the ballast attachment block, the first depth releasemechanism comprising a piston disposed within a cylinder defined in theballast attachment block, the piston defining an outer face exposed toambient pressure; a link member coupled to the piston and to the firstballast weight, wherein at depths in water above a predetermined depththe link member holds the piston in a non-triggered state; and whereinat the predetermined depth and below ambient pressure acting on theouter face of the piston creates a tension force that breaks the linkmember and releases the first ballast weight from the first attachmentpoint.
 16. The system of claim 15 wherein the link member defines acentral axis, the piston defines a central axis, and wherein the centralaxis of the link member is coaxial with the central axis of the piston.17. The system of claim 15 wherein the link member is a copper-basedalloy.
 18. The system of claim 15: wherein the link member is brass andhas at least one portion defining an outside diameter of about 0.2centimeter (cm); and wherein the piston an outside diameter of about 2.5cm.
 19. The system of claim 15 wherein the predetermined depth is about55 meters.
 20. The system of claim 15 further comprising: a counter borewithin the first ballast weight; and an extension portion of the pistondefined on the distal end of the piston, and in the non-triggered statethe extension portion of the piston is telescoped within the counterbore of the first ballast weight; wherein at the predetermined depth andbelow ambient pressure acting of the face of the piston retracts theextension portion out of the counter bore of the first ballast weight.